Dynamic Molecular Behavior on Thermoresponsive Polymer Brushes

The surface dynamics of individual surfactant and polymer molecules on thermally responsive polymer brushes (poly­(<i>N</i>-isopropylacrylamide), PNIPAAM) were studied using high throughput single molecule tracking microscopy. The probe molecules universally exhibited intermittent hopping motion, in which the diffusion switched between mobility and confinement with a broad distribution of waiting times; this was analyzed in the context of a continuous time random walk (CTRW) model described using “waiting time” and “flight length” distributions. We found that the surface mobility, which was affected by waiting times and flight lengths, of both probe molecules increased abruptly with temperature above the 32 °C lower critical solution temperature (LCST) transition of the PNIPAAM brush. In particular, above the LCST, where the polymer brush collapsed into a more hydrophobic dense polymer film, the effective diffusion coefficients and mobile fraction of probe molecule increased, suggesting that mobility was inhibited by penetration into the brush at lower temperatures. Waiting times at lower temperature were twice as long as at higher temperatures, and the longest flight length increased from 0.9 to 1.8 μm. Moreover, we found that the high density of strong binding sites available on the swollen PNIPAAM brush led to long waiting times and a high probability of readsorption, which resulted in short flight lengths, while the absence of strong binding sites on collapsed PNIPAAM films led to short waiting times and long flights